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LOCAL anesthetics can provide effective corneal anesthesia during corneal epithelial wound healing. Prolonged (or even a single) application of local anesthetics, however, causes delay of corneal reepithelialization after wounding. 1,2 Although topical lidocaine results in dose-dependent impairment of corneal epithelial wound healing, 3 the mechanism by which lidocaine negatively alters wound healing is not known.

Epidermal growth factor (EGF) is generally considered to be the main effector during corneal wound healing. 3–6 Epidermal growth factor receptor (EGFR) is a tyrosine kinase receptor present on the surface of most epithelial cells. Binding of EGF to the extracellular region of EGFRs leads to their dimerization and autophosphorylation of specific tyrosine residues of the intracellular activation region, and then results in activation of multiple downstream signaling pathways, leading to cell growth and proliferation. 7

In the previous studies, 8–11 we showed that clinical concentration of lidocaine interacts with the aromatic ring of phenylalanine (F) and tyrosine (Y), with the negatively charged acidic amino acids, aspartic acid (D) and glutamic acid (E), and with the basic amino acids, lysine (K) and arginine (R). Lidocaine could bind around the sodium channel inactivation gate (D
I F
MT EE
1487–1493) followed by sodium channel block, 8,9 and also could bind around an autophosphorylation site of insulin receptor (RD
I Y1158E
T DY1162Y1163R
) followed by the inhibition of its tyrosine kinase activity (table 1). 10,11 Inspection of the amino acid sequence of autophosphorylation sites of EGFR 12 showed that several autophosphorylation sites are the plausible binding sites for lidocaine (EEKEY845HA E
, D
A DEY992LI, RD
PH Y1105Q D, D
NP DY1148QQ DFF
, E
NA EY1173L R
) (table 1). Therefore, we hypothesized that lidocaine interacts directly with the intracellular activation region of EGFR and then suppresses EGF signaling. To investigate the effect of lidocaine on autophosphorylation of EGFR, we used purified EGFR. We also studied changes in the proliferation of SV40-immortalized human corneal epithelial cells (HCECs) 13 by application of lidocaine into the cultured HCECs and evaluated the effect of lidocaine on tyrosine phosphorylation of EGFR in the HCECs.

Table 1. Hypothetical Binding Sites for Lidocaine in the Sodium Channel Inactivation Gate and Autophosphorylation Sites of Several Tyrosine Kinases

The cells were detached with a 0.25% trypsin/1 mm EDTA-4Na solution for 20 min. Subsequently, the cells were seeded onto 96-well plates (1 × 103cells/well), and test compounds were added to the wells (100 μl in total for each well). Control medium contained Dulbecco’s modified Eagle’s medium, 1% (vol/vol) fetal bovine serum, penicillin, streptomycin, choleratoxin, and insulin. Test compounds consisted of the control medium and drugs of various concentrations (10 ng/ml EGF and 4 μM, 40 μM, 400 μM, 4 mM, and 40 mm lidocaine). Eight wells each were used for control and each drug concentration. Previous studies showed that 4 –40 ng/ml EGF was appropriate to induce cell proliferation and tyrosine phosphorylation in a cell culture study using corneal epithelial cells. 15,16

After incubation in a 37° C, 5% CO2environment for 5 days, the number of cells in each well was measured colorimetrically by CellTiter 96®AQueous One Solution Cell Proliferation Assay (Promega Corporation, Madison, WI) according to the manufacturer’s protocol. Briefly, 20 μl CellTiter 96®AQueous One Solution Reagent was added into each well. After 1 h of incubation, the absorbance of each well at 492 nm was recorded by a 96-well plate reader (Multiskan BICHROMATIC; Labsystems, Helsinki, Finland). After subtraction of the background absorbance (i.e.
, the average absorbance of wells containing medium without any cells), the ratios of the absorbance can be interpreted as those of cell number of each well.

Analysis of the Effect of Lidocaine on Autophosphorylation of EGFR on Cultured HCEC

Data were analyzed by one-way analysis of variance with Bonferroni-corrected post hoc
analysis. The statistical significance was established at the level of P
< 0.05. All values are reported as mean ± SD.

Fig. 2. Cell proliferation of human corneal epithelial cell in the presence or absence of lidocaine. The cells were incubated for 5 days with epidermal growth factor (EGF), lidocaine, or both. Solid bars
represent basal and EGF-stimulated levels of culture density, which is a percent of control absorbance at EGF-stimulated level without lidocaine (100%). *P
< 0.05, **P
< 0.01 versus
EGF-stimulated level without lidocaine; n = 4 for each lane.

Fig. 2. Cell proliferation of human corneal epithelial cell in the presence or absence of lidocaine. The cells were incubated for 5 days with epidermal growth factor (EGF), lidocaine, or both. Solid bars
represent basal and EGF-stimulated levels of culture density, which is a percent of control absorbance at EGF-stimulated level without lidocaine (100%). *P
< 0.05, **P
< 0.01 versus
EGF-stimulated level without lidocaine; n = 4 for each lane.

To assess tyrosine phosphorylation of EGFR in HCECs, equal amounts of protein from HCECs were subjected to immunoprecipitation with anti-EGFR antibody followed by immunoblotting with antiphosphotyrosine antibody or anti-EGFR antibody. EGF stimulation resulted in a marked increase in tyrosine phosphorylation of EGFR in HCECs. EGF-stimulated responses of EGFR were taken as 100% (fig. 3). Lidocaine (400 μM, 4 mM, and 40 mM), however, significantly attenuated EGF-stimulated tyrosine phosphorylation of EGFR (44.6 ± 26.1, 21.4 ± 12.9, and 21.7 ± 10.9%, respectively) relative to the control level (100%). EGFR protein levels did not differ among every lane.

Discussion

Epidermal growth factor exists as a constant component of human tear fluid, 17 and both messenger RNA (mRNA) for EGF and mRNA for EGFR are present in HCECs. 18 EGF is a potent mitogen for corneal epithelial proliferation. 7 Therefore, the current study suggests that the attenuation of biologic functions of EGF by lidocaine is attributable to the inhibitory effect on the proliferation of HCECs.

Local anesthetic lidocaine is expected to interact with acidic, basic, and aromatic amino acids around several autophosphorylation sites in the EGFR by a variety combinations of noncovalent interactions, such as electrostatic, π-π stacking,19 cation-π,20 C-H-π,21 and aromatic C-H···O hydrogen bonding22 interactions. The π-π stacking provides interactions between the aromatic ring of lidocaine and aromatic amino acids (F and Y).19 The aromatic ring of lidocaine can interact with the side chains of basic amino acids (K and R) through cation-π interaction.20 Tertiary amine nitrogen of lidocaine interacts electrostatically with any of the negatively charged acidic amino acids (D and E). We observed that several autophosphorylation sites of tyrosine residues in EGFR are surrounded by acidic, basic, or aromatic amino acids. Moreover, lidocaine is an amphiphilic molecule and forms micelles. Therefore, it can be considered that lidocaine binds to tyrosine residues as a micelle by interacting with tyrosine itself and/or with acidic, basic, or aromatic amino acids by noncovalent interactions (table 1). Taking these facts together, we suggest that lidocaine inhibits EGF-stimulated tyrosine kinase activity of EGFR through the interaction with tyrosine residues themselves or with the residues residing around them in the autophosphorylation sites of the EGFR.

Kuroda et al.8,9 studied interactions between local anesthetics and sodium channel inactivation gate–related peptides, and reported that local anesthetics, dibucaine and lidocaine, interact with the aromatic ring of phenylalanine (F1489) and with the negatively charged amino acids (D1487, E1492) around the sodium channel inactivation gate (DIFMTE 1487–1492). On the other hand, etidocaine is known to bind with aromatic amino acids, phenylalanine (F1764), and tyrosine (Y1771) in the IVS6 segment facing the pore of the sodium channel. 23 Therefore, in the previous study, we suggested that local anesthetics interact both with the inactivation gates and with the S6 segments in the sodium channel. 11 We found the close resemblance between the amino acid sequence around the sodium channel inactivation gate and that around the autophosphorylation site of insulin receptor (table 1) 10,11 and reported that lidocaine interacted with this site of insulin receptor. 10 Although there are no homologous alignments in hypothetical binding sites for lidocaine among EGFR, sodium channel, and insulin receptor in table 1, lidocaine would bind to autophosphorylation sites of EGFR, which are surrounded by acidic, basic, and aromatic amino acids, with noncovalent interactions. Other local anesthetics, which are amines having aromatic rings, can also interact with these hypothetical binding sites.

Various kinds of growth factors, such as EGF, keratinocyte growth factor, insulin-like growth factor, fibroblast growth factor, transforming growth factor, hepatocyte growth factor, and platelet-derived growth factor, play a key role in corneal wound healing. 6,7,24 Autophosphorylation sites of both keratinocyte growth factor and insulin-like growth factor receptors are surrounded by acidic, basic, or aromatic amino acids (EEY770L D
L in keratinocyte growth factor receptor, RD
I Y1131E
T DY1135Y1136R
in insulin-like growth factor receptor). 25,26 Insulin, which was contained in our medium for the HCEC study, may also play a mitogenic role on corneal epithelial cells through insulin receptor, 27 and its autophosphorylation site is surrounded by these amino acids (RD
I Y1158E
T DY1162Y1163R
). We suggest that lidocaine interacts not only with EGFR but also with keratinocyte growth factor receptor and insulin-like growth factor receptor in addition to insulin receptor on the surface of HCECs (table 1). Moreover, the limitation of the current study is that we could not exclude the possible mechanisms of the inhibitory effect of lidocaine either upstream of the tyrosine phosphorylation sites of EGFR, such as the EGF/EGFR ligand binding site, or at downstream sites. Although autophosphorylation site of EGFR is an important target of lidocaine, other possible sites than EGFR would also play a role in the suppression of HCEC proliferation.

Clinical application of lidocaine, which ranges in concentration from 0.5 to 2% (approximately 20 – 80 mM), is associated with delay of corneal reepithelialization after corneal wounding. 1,2 Instillation of these concentrations of lidocaine, however, does not provide a measurable steady state concentration because it is diluted and washed away. To evaluate the effect of steady state lidocaine concentrations on corneal epithelial wound healing, Bisla and Tanelian 3 performed a tissue culture study using rabbit cornea with subepithelial wounds, and reported that continuous perfusion of 250 μg/ml (approximately 1 mM) lidocaine for 75 h delayed wound healing, but 100 μg/ml (approximately 400 μM) lidocaine did not. In the current study, 400 μm lidocaine suppressed HCEC proliferation slightly, but 40 μm lidocaine did not. We suggest that prolonged application of a low concentration of lidocaine (< 400 μM) can be used safely on the cornea.

High concentrations of lidocaine (4 and 40 mM) showed greatly reduced survival of HCECs in the current study. Tissue culture study using rabbit cornea also showed that 500 and 1,000 μg/ml (approximately 2 and 4 mM) lidocaine completely halted reepithelialization during corneal wound healing. 3 In cell culture studies, lidocaine (> 3 mM) induced apoptosis and necrosis in both dose-dependent and time-dependent manners. 28–30 Taken together, these results show that prolonged application of high concentrations of lidocaine induces cell death. The inhibitory effect of lidocaine on EGFR might not be a single cause of the halt in cell proliferation. Other mechanisms, which induce apoptosis or necrosis in cultured HCECs, might also be causes of the effect of high-concentration (4 and 40 mM) lidocaine on HCEC proliferation.

In conclusion, lidocaine directly inhibits tyrosine kinase activity of EGFR. This mechanism may be one of the causes of corneal toxicity of topical lidocaine after corneal wounding.

Fig. 2. Cell proliferation of human corneal epithelial cell in the presence or absence of lidocaine. The cells were incubated for 5 days with epidermal growth factor (EGF), lidocaine, or both. Solid bars
represent basal and EGF-stimulated levels of culture density, which is a percent of control absorbance at EGF-stimulated level without lidocaine (100%). *P
< 0.05, **P
< 0.01 versus
EGF-stimulated level without lidocaine; n = 4 for each lane.

Fig. 2. Cell proliferation of human corneal epithelial cell in the presence or absence of lidocaine. The cells were incubated for 5 days with epidermal growth factor (EGF), lidocaine, or both. Solid bars
represent basal and EGF-stimulated levels of culture density, which is a percent of control absorbance at EGF-stimulated level without lidocaine (100%). *P
< 0.05, **P
< 0.01 versus
EGF-stimulated level without lidocaine; n = 4 for each lane.